1. Field of the Invention
The present invention relates to a vehicle environment monitoring device usable for performing monitoring for safety when a two-wheel vehicle such as a motorbike or a bicycle, or a four-wheel vehicle such as an automobile, which has the vehicle environment monitoring device mounted thereon moves; for example, usable for, when such a vehicle moves right or left for the purpose of turning left or turning left or for changing lanes, notifying the driver of the vehicle by means of an alarm that there is a danger of the vehicle contacting or colliding against another vehicle present in the environment thereof and thus supporting safety confirmation; a vehicle environment monitoring method using the same; a control program for causing a computer to carry out the vehicle environment monitoring method; and a computer-readable recording medium having the control program recorded thereon.
2. Description of the Related Art
Recently, the increase of traffic accidents such as contact or collision of automobiles or other vehicles is a serious social problem. Especially, contact or collision often occurs when the vehicle moves right or left for changing lanes, turning right, or turning left although the driver pays attention. Many contact or collision accidents are considered to occur for the reasons of, for example, (i) the driver of the vehicle cannot sufficiently check the environment behind and beside the vehicle using a side mirror or the like, and (ii) there are other vehicles at the so-called blind spot which is not reflected on the side mirror or the like.
In order to prevent such traffic accidents, for example, Japanese Laid-Open Publication No. 2000-285245 discloses a vehicle collision prevention device including notification means for taking images of the environment behind the vehicle by a camera mounted on the vehicle, detecting the presence of another vehicle from the taken images, alarming and notifying the driver that another vehicle is approaching when there is a danger of the vehicle contacting or colliding against the another vehicle coming from behind when the vehicle is moving right or left for the purpose of changing lanes, turning right or turning left.
With this vehicle collision prevention device, from each of a plurality of frame images which were taken in time series by imaging means, straight line edges which form a white line present on the road on which the vehicle is running or a ridge line of the road, and an extinction point (infinite far point) which is an intersection of the straight line edges are detected. Based on the information on the positions of the white line, the ridge line, and the extinction point, a detection area is restricted. A moving amount between frame images of a small area (block) which is obtained by dividing the detection area is calculated by a technique referred to as a “optical flow”. In this case, the detection area is area a represented by oblique lines in FIG. 11, which is about a half of the image plane. Information on the moving amount which is calculated in this manner is used for detecting, for example, another vehicle which is approaching rapidly from behind the vehicle so as to pass the vehicle.
However, the conventional vehicle collision prevention device disclosed in the above-identified publication has the following problems.
A white line is detected in order for checking the lane while the vehicle is running, and an extinction point is obtained for finding a moving direction of an object. Moreover, an optical flow is used for checking how each block obtained by dividing the detection area moves. Therefore, highly sophisticated calculations are required. In addition to the problem regarding the quality of such highly sophisticated calculations, the calculation amount is very large because the image area E represented by the oblique lines in FIG. 11 is as large as about a half of the image plane.
Because the calculation amount is very large, real-time processing is impossible. In order to compensate for this, a method of, for example, calculating a moving amount by unit time for estimation is used. This makes it impossible to accurately detect the moving direction of another vehicle, and errors are likely to occur. In addition, there are other problems that, for example, some roads do not have white lines and therefore the moving amount of the object cannot be calculated. For these reasons, the above-described vehicle collision prevention device is not practical.